The global distribution of chorus wave amplitudes and their wave normal angles is investigated using high‐resolution wave spectra and waveform data from THEMIS for lower‐band and upper‐band chorus ...separately. Statistical results show that large amplitude chorus (>300 pT) occurs predominantly from premidnight to postdawn and is preferentially observed at lower L shells (<8) near the magnetic equator. However, strong or moderate chorus extends further into the afternoon sector and to higher L shells. For lower‐band chorus, strong waves (>50 pT) tend to have wave normal angles of <20° and their wave normal angles become even smaller with increasing wave amplitudes. For modest waves, the wave normal angles are distributed over a broad range with a major peak at <20° and a secondary peak at 60°–80°. Wave normal angles of lower‐band chorus are generally smaller on the dayside than on the nightside possibly due to the more uniform and more compressed magnetic field configuration on the dayside. Lower‐band chorus becomes more oblique with increasing latitude on the dayside, whereas on the nightside the probability of observing oblique chorus decreases at higher latitudes. Compared to lower‐band chorus, the properties of upper‐band chorus are somewhat different. Upper‐band chorus is considerably weaker in magnetic wave amplitudes, shows tighter confinement to the magnetic equator (<10°), and occurs at smaller L shells (<8). Furthermore, wave normal angles of upper‐band chorus are generally larger than those of lower‐band chorus, but the occurrence rate still peaks at wave normal angles of <20°, particularly for strong upper‐band chorus.
Key Points
Large amplitude chorus occurs from premidnight to dawn near the equator
Strong lower‐band chorus typically has small wave normal angles (less than 20 deg)
Upper‐band chorus is more oblique and much weaker than lower‐band chorus
Quantifying radiation belt precipitation and its consequent atmospheric effects requires an accurate assessment of the pitch angle distribution of precipitating electrons, as well as knowledge of the ...dependence of the atmospheric deposition on that distribution. Here Monte Carlo simulations are used to investigate the effects of the incident electron energy and pitch angle on precipitation for bounce period time scales, and the implications for both the loss from the radiation belts and the deposition in the upper atmosphere. Simulations are conducted at discrete energies and pitch angles to assess the dependence on these parameters of the atmospheric energy deposition profiles and to estimate the backscattered particle distributions. We observe that the atmospheric response is both energy and pitch angle dependent. These effects together result in an energy‐dependent bounce loss cone angle, which can vary by 2–3° with particle energy when considered at low‐Earth orbit. This modeling also predicts that a significant fraction of the input electron distribution will be backscattered and should be observable by low‐Earth‐orbiting satellites as field‐aligned beams emerging from the atmosphere at energies lower than the input distribution and having pitch angles distributed just inside the loss cone.
Plain Language Summary
Energetic particles from the radiation belts are lost to the upper atmosphere through precipitation. This precipitation affects loss rates from the radiation belts as well as atmospheric chemistry and dynamics. In this paper we show that the pitch angle of these particles affects their atmospheric interaction; however, this pitch angle is currently not easy to measure and must be modeled. A key result of this study is that the commonly assumed altitude where radiation belt particles are lost—100 km altitude in the atmosphere—is actually energy dependent, and we quantify this dependence.
Key Points
We characterize energy deposition and atmospheric backscatter of radiation belt electrons as a function of energy and pitch angle
We use these simulations to characterize the bounce loss cone and show that it is energy dependent
The simulated backscatter of precipitation is characterized by field‐aligned beams of low energies which should be observable
Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local ...electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth's outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.
We examine the relationship between earthquakes and ultralow frequency (ULF) wave activity in the nighttime ionosphere based on the electric field data in the direct current/ULF range observed by the ...DEMETER satellite over a ~5.5 year period from May 2005 to November 2010. ULF wave activity is identified by an automatic detection algorithm and those which occur on the geomagnetic disturbed days (Kp > 3 at any time intervals) are discarded. Only the earthquakes with depth ≤70 km and occurring in the region of |MLat| < 40° are selected. A superposed epoch analysis is performed to study the statistical association between ULF wave activity and the selected earthquakes. The results show that (1) there are clearly temporal and spatial correlations between ULF wave activity and earthquakes whose catalog magnitudes are both ≥4.8 and ≥5.0, and (2) enhanced ULF wave occurrence rate happens ~1 day and 1 week before the earthquakes and at less than 200 km distance from the epicenters.
Key Points
Earthquakes correlate closely with ULF wave activity in the nighttime ionosphere
ULF wave activity in the nighttime ionosphere enhances some hours before earthquakes
Pre‐earthquake ULF wave activity increases in an area with a radius less than 200 km
Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with Tperp > Tpara. They are believed to play an important role in the dynamics of ...the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (Pdyn) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L‐shell, magnetic local time (MLT), Pdyn, AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12–15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < −50 nT showed that the storm‐time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions.
Key Points
Dayside outer magnetosphere is a preferential location for EMIC waves
Dayside EMIC occurrence rate is controlled by solar wind pressure
Storm‐time EMIC occurrence in the inner magnetosphere remains low
This brief technique paper presents a method of reconstructing the global, time‐varying distribution of some physical quantity Q that has been sparsely sampled at various locations within the ...magnetosphere and at different times. The quantity Q can be essentially any measurement taken on the satellite including a variety of waves (chorus, hiss, magnetosonic, and ion cyclotron), electrons of various energies ranging from cold to relativistic, and ions of various species and energies. As an illustrative example, we chose Q to be the electron number density (inferred from spacecraft potential) measured by three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes between 2008 and 2014 and use the SYM‐H index, taken at a 5 min cadence for the 5 h preceding each observed data point as the main regressor, although the predictor can also be any suitable geomagnetic index or solar wind parameter. Results show that the equatorial electron number density can be accurately reconstructed throughout the whole of the inner magnetosphere as a function of space and time, even capturing the dynamics of elementary plasmaspheric plume formation and corotation, suggesting that the dynamics of various other physical quantities could be similarly captured. For our main model, we use a simple, fully connected feedforward neural network with two hidden layers having sigmoidal activation functions and an output layer with a linear activation function to perform the reconstruction. The training is performed using the Levenberg‐Marquardt algorithm and gives typical RMS errors of ~1.7 and regression of >0.93, which is considered excellent. We also present a discussion on the different applications and future extensions of the present model, for modeling various physical quantities.
Key Points
We present a general method that can be used to model any quantity in the inner magnetosphere
We demonstrate our method on the spatiotemporal distribution of electron density
This model could be immensely useful for space weather and other applications
The chorus wave properties are evaluated using Van Allen Probes data in the Earth's equatorial magnetosphere. Two distinct modes of lower band chorus are identified: a quasi‐parallel mode and a ...quasi‐electrostatic mode, whose wave normal direction is close to the resonance cone. Statistical results indicate that the quasi‐electrostatic (quasi‐parallel) mode preferentially occurs during relatively quiet (disturbed) geomagnetic activity at lower (higher) L shells. Although the magnetic intensity of the quasi‐electrostatic mode is considerably weaker than the quasi‐parallel mode, their electric intensities are comparable. A newly identified feature of the quasi‐electrostatic mode is that its frequency peaks at higher values compared to the quasi‐parallel mode that exhibits a broad frequency spectrum. Moreover, upper band chorus wave normal directions vary between 0° and the resonance cone and become more parallel as geomagnetic activity increases. Our new findings suggest that chorus‐driven energetic electron dynamics needs a careful examination by considering the properties of these two distinct modes.
Key Points
Chorus wave normal and spectral properties are evaluated using Van Allen Probes wave data
Lower band chorus has two distinct modes: a quasi‐parallel mode and a quasi‐electrostatic mode
Quasi‐electrostatic chorus occurs preferentially during quiet times, at lower L, and in higher frequencies
The rapid loss of radiation belt electrons in the main phase of geomagnetic storms is believed to be aided by EMIC waves, and is usually analyzed with quasi‐linear theory. However, even moderate EMIC ...wave intensities easily cause resonant electrons to respond nonlinearly, with drastically different results. We map out the region of nonlinear behavior with a single parameter, and show that both the direction and magnitude of scattering can be estimated by analytical expressions. The nonlinear interactions typically lead to advection toward large pitch angles, rather than diffusion toward the loss cone. This is expected to reduce the overall loss rate and greatly affect the distribution of trapped electrons.
We use measurements from NASA's Van Allen Probes to calculate the decay time constants for electrons over a wide range of energies (30 keV to 4 MeV) and
L values (
L = 1.3–6.0) in the Earth's ...radiation belts. Using an automated routine to identify flux decay events, we construct a large database of lifetimes for near‐equatorially mirroring electrons over a 5‐year interval. We provide the first accurate estimates of the long decay timescales in the inner zone (
∼100 days), which are highly resolved in energy and free from proton contamination. In the slot region and outer zone, we compare our lifetime calculations with prior empirical estimates and find good quantitative agreement (lifetimes
∼1–20 days). The comparisons suggest that some prior estimates may overestimate electron lifetimes between
L≈ 2.5–4.5 due to instrumental effects and/or background contamination. Previously reported two‐stage decays are explicitly demonstrated to be a consequence of using integral fluxes.
Plain Language Summary
The Earth is surrounded by two invisible, donut‐shaped belts of charged particle radiation (think electrons and protons) called the Van Allen belts. The particles in these belts orbit rapidly around the Earth in the same region where spacecraft fly, like GPS and weather satellites. Since the particles in the belts can damage satellites, we need to understand what specific processes make the intensity of the belts go up and down. Knowing which processes are important for changing the belt intensity helps us build better computer models that can be used to predict the future state of the belts (much like weather prediction models). This letter uses spacecraft observations to estimate the loss timescales in the radiation belt region, which are then used in a companion paper to better understand the processes that make the belt intensity go down.
Key Points
A large database of radiation belt electron decay timescales is calculated from Van Allen Probes MagEIS measurements
We provide the first accurate estimates of these timescales over a wide range of energies in the inner zone, free from proton contamination
Outer zone decay timescales generally agree well with prior estimates; some differences exist and may be due to instrumental effects
Resonant interactions between electrons and chorus waves are responsible for a wide range of phenomena in near‐Earth space (e.g., diffuse aurora and acceleration of > 1 MeV electrons). Although ...quasi‐linear diffusion is believed to be the primary paradigm for describing such interactions, an increasing number of investigations suggest that nonlinear effects are also important in controlling the rapid dynamics of electrons. However, present models of nonlinear wave‐particle interactions, which have been successfully used to describe individual short‐term events, are not directly applicable for a statistical evaluation of nonlinear effects and the long‐term dynamics of the outer radiation belt, because they lack information on the properties of intense (nonlinearly resonating with electrons) chorus waves. In this paper, we use the Time History of Events and Macroscale Interactions during Substorms and Van Allen Probes data sets of field‐aligned chorus waveforms to study two key characteristics of these waves: effective amplitude
ℬw (nonlinear interaction can occur when
ℬw>2) and wave packet length β (the number of wave periods within it). While as many as 10–15% of chorus wave packets are sufficiently intense (
ℬw>2–3) to interact nonlinearly with relativistic electrons, most of them are short (β < 10) reducing the efficacy of such interactions. Revised models of nonlinear interactions are thus needed to account for the long‐term effects of these common, intense but short chorus wave packets. We also discuss the dependence of
ℬw, β on location (MLT and L‐shell) and on the properties of the suprathermal electron population.
Key Points
The occurrence rate of lower‐band chorus waves interacting nonlinearly with electrons is estimated
Statistics on the fraction of intense lower‐band chorus wave packets are given
The important role of suprathermal electrons for intense chorus wave packet characteristics is shown
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